1. Field of the Invention
The present invention relates to a Viterbi equalizer for use in a mobile telephone or the like, for example.
2. Description of the Prior Art
Heretofore, the development of digital mobile telephones has advanced in U.S., European countries and Japan. A mobile communication system such as the mobile telephone or the like is affected by a so-called multipath propagation because a highrise building or the like is located between a high-speed mobile station such as an automobile and a base station, so that a transmission characteristic between the base station and the mobile station is deteriorated considerably. Therefore, a data transmission with less error is impossible. In addition, such an equivalent transmission characteristic fluctuates with time.
In the above-mentioned mobile communication system, in order to realize a reception with less error, an equalizing technique for correcting the above transmission characteristic becomes indispensable.
Japanese laid-open patent publication No. 4-183041 describes as a conventional equalizing technique a Viterbi equalizer in which transmission data is decoded by using a transmission characteristic provided between the base station and the mobile station on the basis of a maximum likelihood sequence estimation.
More specifically, according to an example of the prior art, in a Viterbi decoder in which a maximum likelihood reception is carried out following a time fluctuation of a characteristic of a transmission line, there is provided a Viterbi decoder which comprises a register for storing a plurality of sample values of a reception signal, a transmission line characteristic calculating circuit for receiving the plurality of sample values from the above register and estimating respective transmission line characteristics relative to a plurality of sequences, a branch metric calculating circuit for calculating distances of the plurality of samples by calculating hypothetical signal reception points relative to the respective sequences from the transmission line characteristics calculated by the transmission line calculating circuit and a Viterbi processor supplied with an output of the branch metric calculating circuit and judging a received signal on the basis of a Viterbi algorithm. According to this conventional Viterbi decoder, however, since a plurality of sample values of the received signal are compared, it cannot be expected that the transmission characteristic between a transmitter and a receiver is estimated accurately.